The cost of energy has continued to steadily rise as power utilities try to cope with continually growing demands, increasing fuel prices, and stricter regulatory mandates. Utilities must also maintain existing power generation and distribution infrastructure, while simultaneously finding ways to add more capacity to meet future needs, both of which add to costs. Burgeoning energy consumption continues to impact the environment and deplete natural resources.
A major portion of rising energy costs is borne by consumers. Increasingly, utilities have begun to adopt complex rate structures that add time-of-use demand and energy charges onto base demand charges to help offset their own costs, such as the costs incurred when power must be purchased from outside energy producers when generation capacities become overtaxed. Consumers still lack the tools and wherewithal to identify the most cost effective ways to appreciably lower their own energy consumption. For instance, no-cost behavioral changes, such as manually changing thermostat settings and turning off unused appliances, and low-cost physical improvements, such as switching to energy-efficient light bulbs, may be insufficient to offset utility bill increases.
As space heating and air conditioning together consume the most energy in the average home, appreciable decreases in energy consumption can usually only be achieved by making costly upgrades to a building's heating and cooling envelope or “shell.” On the other hand, recent advances in thermostat technologies provide an alternative to shell upgrades by facilitating energy efficient use of heating, ventilating, and air conditioning (HVAC) systems. Existing programmable thermostats typically operate an HVAC system based on a schedule of fixed temperature settings. Newer “smart” thermostats, though, are able to factor in extrinsic considerations, such as ambient conditions and consumer use patterns, to adapt HVAC system operation to actual conditions and occupant comfort needs, which in turn helps lower overall energy consumption.
HVAC energy costs are driven by HVAC system use that, in turn, is directly tied to a building's total thermal conductivity UATotal. A poorly insulated home or a leaky building will require more HVAC usage to maintain a desired interior temperature than would a comparably-sized but well-insulated and sealed structure. Reducing HVAC energy costs, though, is not as simple as manually choosing a thermostat setting that causes an HVAC system to run for less time or less often. Rather, numerous factors, including thermal conductivity, HVAC system efficiency, heating or cooling season durations, and indoor and outdoor temperature differentials all weigh into energy consumption and need be taken into account when seeking an effective yet cost efficient HVAC energy solution.
Conventionally, an on-site energy audit is performed to determine a building's thermal conductivity UATotal. A typical energy audit involves measuring the dimensions of walls, windows, doors, and other physical characteristics; approximating R-values of insulation for thermal resistance; estimating infiltration using a blower door test; and detecting air leakage using a thermal camera, after which a numerical model is run to solve for thermal conductivity. The UATotal result is combined with the duration of the heating or cooling season, as applicable, over the period of inquiry and adjusted for HVAC system efficiency, plus any solar (or other non-utility supplied) power savings fraction. The audit report is often presented in the form of a checklist of corrective measures that may be taken to improve the building's shell and HVAC system, and thereby lower overall energy consumption. Nevertheless, improving a building's shell requires time and money and may not always be practicable or cost effective, especially when low- or no-cost solutions have yet to be explored.
Therefore, a need remains for a practical model for determining actual and potential energy consumption for the heating and cooling of a building.
A further need remains for an approach to making improvements in HVAC system energy consumption through intelligent control over system use.